This year we focused on analyzing the expression of the stimulatory G protein, (Gs), in the cochlea. Using a polyclonal rabbit antisera specific for Gs, we localized Gs in the stria vascularis and in the organ of Corti of guinea pigs at the ultrastructural level by using the post- embedding immunogold labeling technique. In the stria vascularis, labeling was intense on the basolateral membrane infoldings of marginal cells and on the juxtaposed membrane of intermediate cells. In contrast, no significant labeling was observed on the luminal membrane of marginal cells. Immunoreactivity was also detected on the cell membranes of various other cells. Membranes associated with Gs in these cells were also often associated with adenylate cyclase. A Gs coupled receptor/Gs/adenylate-cyclase-mediated mechanism is proposed to play an important functional role in processes such as ionic transport across these cells, as supported by previous biochemical and pharmacological evidence. In the organ of Corti, immunoreactivity was abundant on the membrane of supporting cells, especially of the reticular lamina which is composed of inner and outer pillar cells and of phalangeal processes of Deiters' cells. Staining was also seen on the membrane of these cells' bodies which surround nerve fibers, basilar fibers, outer spiral fibers and afferent nerve endings of outer hair cells. In contrast, virtually no gold decorated the membrane of outer hair cells and inner hair cells. Gs may be involved in the regulation of micromechanics in the organ of Corti, but it is unlikely that they are involved in fast sound mechanoreception in hair cells.